Assessing Chemical Insecticide Susceptibility/Resistance Profiles of Malaria Vectors in Three Districts of the Upper West Region of Ghana, and their Public Health Implications.

Abstract

Malaria remains a major public health concern in Ghana, including the Upper West Region, where the intensity of its transmission persists at high levels. The increasing emergence of insecticide resistance in Anopheles gambiae sensu lato vector populations is a major threat to the effectiveness of malaria vector control strategies. The current study assessed the susceptibility of malaria vector populations in Wa, Nadowli, and Lawra districts to older classes of key synthetic chemical insecticides, including pyrethroid (deltamethrin and permethrin), carbamate (bendiocarb), and organophosphate (pirimiphos-methyl); as well as a newer class: neonicotinoid (clothianidin). The evaluation of insecticide resistance in the malaria vector population was conducted using the WHO susceptibility bioassay guidelines and kits. Mosquito larvae were collected from selected breeding sites and reared to adults. Overall, 300 female Anopheles gambiae s.l. were exposed to varying concentrations of insecticides. Knockdowns and mortalities were recorded. Results revealed considerable variability in susceptibility across sites and insecticide classes. Knockdown rates (KDRs) ranged from 4.0% to 16.0% at lower deltamethrin concentrations but reached 100% at higher doses. Higher KDT₅₀ and KDT₉₅ values were estimated for wild mosquito populations compared with the standard Kisumu susceptible strain. The highest KDTs were calculated for the pyrethroid-based bioassays, with resistance ratios for KDT50 >5. This suggests significant physiological resistance in the wild mosquito population. For pyrethroid-based bioassays, mortality rates ranged from 20.0% for 0.05% deltamethrin to 100% for 0.50% deltamethrin. In addition, mortality rates for the permethrin ranged from 38.0% to 99.0%. For the carbamate and organophosphate bioassays, respectively, 0.10% bendiocarb induced mortalities ranging from 59.0% to 84.0% while pirimiphosmethyl achieved higher rates (87-99%). However, 2% clothianidin-based bioassays yielded 100% mortality within 24 hours. These findings suggest considerable resistance in the wild An. gambiae s.l. populations to the older classes of insecticides but high susceptibility to the newer insecticide class, justifying the choice of indoor residual spraying insecticide formulation that includes clothianidin for the Upper West Region.